1. Field of the Invention
The present invention relates to a position controller for controlling an electric motor, such as a dc motor, an induction motor or a synchronous motor, for driving a mechanism, such as the work table of a machine tool or the robot arm of an electric industrial robot.
2. Description of the Prior Art
FIG. 1 is a block diagram of a prior art position controller presented in the Symposium for Small Motor Technology, Session B-4 held in 1987 under the sponsorship of the Japan Management Association. Referring to FIG. 1, there are shown a dc motor 1a, a speed detector 4a, a position detector 4b, a subtracter a, a position control circuit 5b, a feed-forward signal generating circuit 5c, an adder 5d, a subtracter 9a, a speed control circuit 9b, a power conversion circuit 13 and a rotational angle command signal generating circuit 15.
In operation, the position control circuit 5b provides a speed signal .omega..sub.ms1. The subtracter 5a subtracts an actual rotational angle signal .theta..sub.m provided by the position detector 4b from a rotational angle command signal .theta..sub.ms provided by the rotational angle command signal generating circuit 15 to give a rotational angle deviation signal .DELTA..theta. (=.theta..sub.ms, -.theta..sub.m) to the rotational angle control circuit 5b. Then, the position control circuit 5b provides a speed command signal .omega..sub.ms1.
Subsequently, the feed-forward speed signal generating circuit 5c provides a feed-forward signal .omega..sub.ms2. The rotational angle command signal .theta..sub.ms provided by the rotational angle command signal generating circuit 15 is given to the feed-forward speed signal generating circuit 5c, and then the feed-forward signal generating circuit 5c performs differentiation to provide the feed-forward speed signal .omega..sub.ms2, namely, the rate of change of the rotational angle command signal .theta..sub.ms.
The adder 5d adds the speed command signal .omega..sub.ms1 and the feed-forward speed signal .omega..sub.ms2 and provides a final speed command signal .omega..sub.ms (=.omega..sub.ms1 +.omega..sub.ms2).
When a control signal is provided by the speed control circuit 9b, the subtracter 9a subtracts the actual speed signal .omega..sub.m provided by the speed detector 4a from the final speed command signal .omega..sub.ms to give a speed deviation signal .delta..omega. (=.omega..sub.ms -.omega..sub.m) to the speed control circuit 9b, and then the speed control circuit 9b provides a torque signal. A control signal is given to the power conversion circuit 13 to control the power conversion circuit 13 so that the output torque of the dc motor 1a conforms to the torque signal.
Since the armature current of the dc motor 1a varies substantially in proportion to the torque, the speed control circuit 9b is provided internally with a current feedback loop to improve the response characteristics of the speed control circuit 9b.
As is generally known, a control system provided with a speed control loop and a current control loop as minor loops to make the position of the output shaft, i.e., rotational angle of the dc motor 1a vary according to a command signal at high response is called a cascaded control system. Such a cascaded control system requires the minor loops to operate at a high response speed. In the prior art position controller shown in FIG. 1 for controlling the position of the output shaft of an electric motor, the order of importance of high-speed response characteristics is the current control loop, the speed control loop and the position control loop. Generally, the gain of the control system is determined so that the response frequency of the current control loop is several times that of the speed control loop, and the response frequency of the speed control loop is several times that of the position control loop.
Accordingly, the response characteristics of the speed control loop needs to be enhanced to enhance the response characteristics of the position control loop. The prior art position controller shown in FIG. 1 is provided additionally with the feed-forward speed signal generating circuit 5c to improve the response characteristics of the position control loop. The feed-forward speed signal generating circuit 5c provides the feed-forward speed signal .omega..sub.ms2 proportional to the rate of change of the position command signal .theta..sub.ms. Since the operating speed .omega..sub.m of the dc motor is the derivative of the rotational angle .theta..sub.m, the rotational angle .theta..sub.m of the dc motor 1a follows the rotational angle command signal .theta..sub.ms when the speed control circuit 9b controls the dc motor 1a so that the actual speed .omega..sub.m of the dc motor 1a coincides with the feed-forward speed signal .omega..sub.ms2.
The foregoing prior art position controller is capable of controlling the position at a high response speed owing to the operation of the feed-forward speed signal generating circuit 5c even when it is difficult to increase the gain of the angular position control circuit due to restrictions placed by the response characteristics of the speed control loop.
However, since the feed-forward speed signal generating circuit 5c determines the feed-forward speed signal by differentiation, the speed changes sharply entailing the sharp change of the output torque of the dc motor 1a if the rotational angle command signal .theta..sub.ms does not change smoothly. If the speed of the dc motor 1a driving, for example, the ball-screw shaft for driving the work table of a machine tool, is controlled in such a mode, a high impulsive force will be applied to the work table, causing the machine tool to generate vibrations and noise.
Furthermore, if the ball-screw shaft serving as a component of a torque transmission member has a relatively low rigidity or the reduction gear of the machine tool has a backlash, large mechanical vibrations are liable to be generated if the response frequency of the speed control loop is increased. Accordingly, the response frequency of the speed control loop when controlling the electric motor driving the work table through a transmission mechanism must be lower than that of the speed control loop when controlling only the electric motor. The moment of force that acts on the robot arm of an electric industrial robot varies according to the position of the robot arm and, when the electric motor is used for driving the robot arm of an electric industrial robot, the response frequency of the speed control loop decreases when the moment of inertia of the robot arm increases. Under such circumstances, the operating speed of the electric motor is unable follow the feed-forward speed signal and the overshooting of response to a rotational angle control command, i.e., a position control command, occurs even if the feed-forward speed signal is determined by the feed-forward speed signal generating circuit, because the response frequency of the speed control loop is low.